1. Technical Field
This invention relates to polymeric material useful in acquiring quantitative surface pressure measurements. More specifically, the invention relates to synthesis of a nano-material which exhibits an optically detectable response to changes in pressure.
2. Description of the Prior Art
Acquisition of global, surface pressure data by optical non-intrusive methods has been sought after for many years. Techniques used for the acquisition of these data range from detection of Raman scattering to materials commonly called pressure sensitive paints. Traditionally, pressure sensitive paints consist of a host matrix in which one of a variety of chromophores is encapsulated. The host matrix is often a polymeric material such as polydimethylsiloxane (PDMS), but other materials such as sol-gels have been used. Typical chromophores used have included platinum octaethylporphyrin (PtOEP) and ruthenium-based complexes. The functionality of these pressure sensitive paints depends on the dynamic quenching of the chromophore's luminescent emission by oxygen. In order for this dynamic quenching to be effective the host matrix must allow the diffusion of oxygen throughout the “paint” to the chromophores. One example of a prior art application requiring the diffusion of oxygen is U.S. Pat. No. 5,965,632 to Gouterman which teaches the use of a pressure sensitive pain incorporating an acrylic and flouroarcrylic polymer binder. A pressure sensing dye is dissolved or dispersed in the polymer matrix. The dyes illuminate in the presence of molecular oxygen. Similarly, in a prior non-related application to Kelley et al., the pressure sensitive material used has a host polymer and a fluorescent compound attached to the host polymer. The host polymer has a “rubber like” characteristic rather than a rubbery elastomer. In addition, Kelley et al. focuses on the use of polystyrene in place of a polyurethane and rubberized polymethacrylate because it does not contain oxygen. Accordingly, one of the limitation of the prior art pressure sensitive paints is the sensitivity to oxygen.
Dynamic quenching by oxygen follows an association known as the Stern-Volmer relationship. This relationship between changes in luminescent emission intensity, I, and the local partial pressure of oxygen, po, is expressed as Io/I=A+B(p/po) where A=ka/(ka+kqpo) and B=kqpo(ka+kqpo). In these equations Io is the incident excitation light intensity, ka is the intrinsic de-excitation rate in the absence of oxygen, kq is the quenching rate due to collisions with oxygen and p is the local pressure. In addition, A+B=1. A typical plot of the relationship between changes in luminescent emission intensity and local partial pressure of oxygen is shown in FIG. 1. Under the conditions normally experienced during high-speed tests (e.g. supersonic), systems following the Stern-Volmer relationship exhibit relatively large changes in emission intensity for only small changes in pressure. However, the same systems used for low-speed (e.g. atmospheric) tests exhibit only extremely small changes in emission intensity even for large changes in pressure. This is shown schematically in FIG. 2 which is a graph showing the Stern-Volmer relationship between small changes in intensity and large changes in pressure. In addition, systems following the Stern-Volmer relationship exhibit decreasing emission intensity with increasing pressure. Accordingly, this results in lower signal to noise ratios with the maximum signal to noise ratio at vacuum, or near vacuum, conditions.
Because these systems rely on oxygen quenching to vary emission light intensity with changes in pressure, any perturbation to the host matrix' oxygen permeability alters the pressure sensitive paint's performance. For example, variations in humidity and/or temperature affect pressure sensitive paint's performance. Unfortunately, even the oils normally found on human skin have been known to affect the performance of some traditional pressure sensitive paint formulations making handling of painted test articles difficult. Accordingly, there is a need for a pressure sensitive material that mitigates sensitivity to oxygen.